The invention relates to door opening alarms. It relates, more particularly, to self-contained alarms powered by batteries and emitting a tone.
There are many instances: in retail establishments, small manufacturing shops, in restricted areas of larger commercial and industrial enterprises; where an audible warning of the opening of a door is required, or desirable. A typical case is a small retail or service shop open to the public in which the employees may be required to spend a substantial portion of the time away from direct view of the entrance door.
The prior art provides many different door alarms, operated mechanically, electrically and pneumatically, which fulfill the above-defined function. Such alarms may be set off by electrical switches integral with door mats, by electric eye--photoelectric--sensing units incorporated in the door frame, by means of pneumatic switches, or, simply consist of bells affixed to the door proper. All of these alarms of the prior art exhibit a common failing, that of being defeatable by a knowledgeable entrant, a would-be thief, for example, who desires to enter without being noticed.
Mat switches, of the electrical and pneumatic varieties, are readily overstepped; photoelectric sensors can be circumvented by avoiding interruption of the signal beam; mechanically coupled devices may be rendered ineffective by a very slow and deliberate operation of the door; electrical signals relying on connection to the building supply net may have their current supply interdicted by cutting a wire prior to entry.
It is, therefore, the primary object of the invention to provide a door alarm device which cannot be circumvented by an entrant to the protected space.
It is further object of the invention to provide for a secure door-opening alarm device which is self-contained and easily installed.
It is yet another object of the invention to provide features in a door alarm device which render it economical in construction and reliable in use.
it is also an object of the invention to provide a door alarm device equally adaptable to doors opening inwardly and/or outwardly on hinges, swing doors and rotary doors.